The present invention relates to a process for improving traction coefficient of traction drive fluid at high temperatures. More particularly, it is concerned with a process for remarkably improving traction coefficient over a wide range of temperatures, especially at high temperatures by using specific compounds as a base stock of traction drive fluid.
A traction drive fluid is a fluid to be used in a traction drive (a friction driving device utilizing rolling contact), such as an automobile continuously variable transmission, an industrial continuously variable transmission, and a hydraulic machine, and needs to have a high traction coefficient and to be stable against heat and oxidation and be inexpensive.
In recent years, there has been the increasing trend toward miniaturization of a traction driving device using such a traction drive fluid and the use of such a device under high-speed, high-load conditions. Thus, it has been desired to develop a traction drive fluid of much higher performance.
In designing a traction driving device, it is generally said that the size of the traction driving device is in inverse proportion to the 0.45 power of a traction coefficient of a lubricant, provided that the traction driving device has the same service life and output ratio. Therefore, as the traction coefficient of a lubricant is higher, the traction driving device can be more reduced in size and weight. In this designing process, there is employed the minimum traction coefficient value in the temperature range within which the driving device is used; i.e., a traction coefficient value at the highest temperature within the foregoing temperature range because as the temperature increases, the traction coefficient value lowers. Therefore, a traction drive fluid having a high traction coefficient even at high temperatures is desirable for the miniaturization and weight-reduction of the traction driving device.
Also from a viewpoint of the use under high-speed, high-load conditions, it has been desired to develop a traction drive fluid having a high traction coefficient even at high temperatures.
Various types of compounds have been proposed as traction drive fluids. Examples are described in, e.g., U.S. Pat. Nos. 3,411,369, 3,440,894, 3,595,796, 3,595,797, 3,597,358, 3,598,740, 4,008,251, 3,645,902, 3,646,233, 3,975,208 and 4,329,529. Although these compounds all have a high traction coefficient at low temperatures (from room temperature to 80.degree. C. (176.degree. F.)), they have disadvantages in that at high temperatures (from 80.degree. to 140.degree. C. (176.degree.-248.degree. F.)) the traction coefficient drops, or although the traction coefficient does not drop, the viscosity is high, resulting in a serious stirring loss. This will lead to a reduction in power transmission efficiency.
Further, U.S. Pat. No. 3,411,369 discloses traction drive fluids comprising fused saturated hydrocarbons. Though the fluids have a high traction coefficient over a temperature range of from room temperature to 200.degree. F., the traction coefficient of the fluids drops seriously at temperatures of more than 250.degree. F. Therefore, these traction drive fluids cannot be used for continuously variable transmission for cars which is driven at high temperatures.
U.S. Pat. No. 3,440,894 discloses a compound having cyclic group as traction drive fluids, and U.S. Pat. No. 3,975,278 discloses hydrogenated compound of .alpha.-methylstyrene dimer as traction drive fluids. These compounds, however, have disadvantages in that the traction coefficient is lowered at high temperatures.